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Assuming I have 2 twisted pairs of a Cat 5e cable (the complete setup is below). Can I run a LAN and an analogue landline phone on these 4 wires? I assume that the LAN is running at 100Mbps and phone at 64Kbps. I realise there may be some interference and lost TCP/IP packets but would it work in theory?

The complete setup is :

  • I have a Cat 5e cable embedded in a concrete wall that runs around the house with an access point in each room (not my work, I inherited it).
  • On 2 twisted pairs I have a cable TV set up which needs 50Mbps for 4K video so 2 twisted pairs will be dedicated to the cable TV.
  • The other 2 twisted pairs I would like to set up as a separate LAN and phone line.

The cable is Cat 5e, it's embedded in concrete walls without any cable raceways so I cannot thread in another cable or change the set up easily. The cable has been set up as a single sequential path, i.e. in the first AP, I see one cable wired up, in each of the other APs I see 2 cables, one coming in from last AP and then one out to next AP. And then finally I see the last AP with just the incoming AP wired up.

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    What kind of landline phone – analog, VoIP, ISDN? And can you clarify whether you want one LAN + landline or two LANs + landline? (Your title says the former, but your main post says the latter.)
    – grawity_u1686
    Commented Jul 15 at 13:30
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    100Mbps ethernet requires 2 twisted pairs, so it needs all 4 of those wires. Phone signals down the same wires will cause issues and I would expect a connection that is constantly dropping and unstable rather than just a few dropped packets.
    – Mokubai ♦
    Commented Jul 15 at 13:58
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    If both rooms have the same electrical net, you could run the LAN using a ethernet over power adapter configuration. That way, the LAN is not using that specific cable anymore, but the electrical grid. You probably get higher speeds too, and with todays systems, its very stable. The network cable you have in place can then be used for other things.
    – LPChip
    Commented Jul 15 at 14:40
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    Wild idea: Because ethernet uses differential pairs, you could theoretically run the analogue phone line as a common mode signal in each pair. For example pair 1 could be the ethernet RX pair on the differential mode, while also being the negative side of the phone line on the common mode. This could be realised by using passive PoE injectors on both ends, but actually feeding in a phone line instead of power
    – 小太郎
    Commented Jul 16 at 0:15
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    I have a wireless landline phone that has a wireless transceiver that can be plugged in where your phone line comes in (within wireless range, which is quite far) and then a charging station which acts as the "base station" where you leave the phone when not in use, and which only requires access to a wall socket. This seems like by far the easiest solution, far easier and cheaper than a VoIP phone or a solution to get ethernet to run over one pair. Unless you insist on a non-wireless landline for some reason.
    – FrederikVds
    Commented Jul 16 at 6:10

2 Answers 2

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On 2 twisted pairs of the internal cables I have a cable tv set up which needs 50Mbps for 4K video so 2 twisted pairs will be dedicated to the cable tv. The other 2 twisted pairs I would like to set up as a separate LAN and phone line.

That won't work – you don't have enough spare pairs for that. With a 4-pair cable you can have either two LANs, or one LAN + landline, but can't fit all three.

Each standard 10/100 Mbps Ethernet connection requires two pairs. If you had two such connections on the cable, you wouldn't have any pairs left for the telephone – yes, you can share a Cat5 cable between Ethernet and analog telephone (and that was where Ethernet's usage of Cat5 comes from in the first place), but not both on the same wires.

(If I understand it correctly, the fact that standard Ethernet is a "baseband" signal – the 'BASE' in 100BASE-TX – means it would conflict with analog telephone on the same wires, as opposed to something like ADSL which uses a "broadband" signal that's modulated on a higher frequency.)

I think your options are:

  • One 10/100 Mbps connection + landline + one unused pair.

    If the TV requires 50 Mbps, that still leaves you with another 50 Mbps – it would be possible to use a pair of managed Ethernet switches to isolate two logical VLANs over the same physical connection, and even to set up QoS so that the TV would have a guaranteed 50 Mbps capacity no matter what the other VLAN is doing.

  • All four pairs for a 1 Gbps Ethernet connection, no analog landline.

    From what I've heard, a pair of old Cisco ATA devices could be used to transform an analog line into VoIP on one end, then back to analog on the other.

    If you can find that kind of equipment, then a 1 Gbps Ethernet connection would give you plenty of capacity for the TV VLAN and your general-purpose VLAN and the VoIP VLAN all at once.

  • One standard 10/100 Mbps Ethernet connection on two pairs + landline + one 100BASE-T1 Ethernet connection on the remaining pair. The latter is a fairly new standard, and it's meant for industrial and automotive Ethernet connections rather than general home use, so it would probably be difficult to find equipment for it – but it's technically a possibility.

  • One 1 Gbps or two 10/100 Mbps Ethernet connections over the cable, and some kind of wireless setup for the landline. I don't know much about this area, but I think I've seen DECT equipment that would make it possible?

  • One 10/100 Mbps Ethernet for the TV + one landline, and wireless LAN from the existing APs – e.g. one of those 'mesh' systems or an 'extender' that lets you connect wired Ethernet equipment to it.

  • MoCA, if the building happens to have coax for cable TV.

  • Power line networking, as mentioned in comments – but in my experience it highly depends on the building and its electric wiring, as I've tried several kinds of HomePlug AV in the past and could never get it to reach even a third of its advertised speed nor stability.

The cable has been set up as a single sequential path, i.e. in the first AP, I see one cable wired up, in each of the other APs I see 2 cables, one coming in from last AP and then one out to next AP. And then finally I see the last AP with just the incoming AP wired up.

That works, but really isn't a nice way to do things... both in terms of reliability and performance. (And requiring APs that have two Ethernet ports, too – many only have one – or an extra switch next to each AP.)

Hopefully at least the APs are all in bridge mode (i.e. actual APs and not whole routers).

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  • Thanks for a great answer! I will take a look at 100BASE-T1 and maybe wireless HDMI and get the cable tv off my LAN.
    – mfc
    Commented Jul 15 at 14:43
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    Another option is to get a pair of VDSL2 ethernet extenders, and you can run up to 320 Mbps data and voice on 1 pair.
    – user71659
    Commented Jul 15 at 22:28
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    ...or converge everything onto IP. Video streaming runs fine over IP, so does voice applications.
    – vidarlo
    Commented Jul 16 at 7:24
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    Does it absolutely have to be an analog landline? Using an ordinary GBit ethernet network (adding switches as necessary) and some VoIP phone without ever converting to analog would be pretty easy, no?
    – Erlkoenig
    Commented Jul 16 at 9:53
  • One 100 Mbit link and two voice pairs, and stick an ethernet switch on the end. OR, just do it right and pull new cat6 cable for data, leaving the existing cat5 as 4 pairs for phone.
    – Criggie
    Commented Jul 16 at 10:40
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Silly, hacky way that can work for short distances:

  • Use the two pairs for 100 Mbps Ethernet
  • Connect the landline phone between the two pairs. Ideally you'd connect to the center tap of the Ethernet transformer at each end, but connecting to one of the lines in each pair can work.

This is based on Ethernet using transformers to electrically isolate the pairs it uses for communication. The main voice frequency of phone communications is low enough that it wouldn't disturb the Ethernet signals.

However, this setup is very likely to cause noise and lost packets on the Ethernet lines when phone is picked up or an incoming call is ringing.

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    I think a surprise 60-100 volts AC at ~25HZ will play merry havoc with ethernet, to the point it could physically damage the switch and the NICs. I mean its tempting to lab that, but not on equipment I'd care about.
    – Criggie
    Commented Jul 16 at 10:42
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    @Criggie Possibly, though ethernet ports regularly see 120 V AC at 50 Hz when one end is powered by ungrounded 240V power supply. That's on all pairs equally though.
    – jpa
    Commented Jul 16 at 16:50
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    @Criggie Should have no damage. Ethernet transformers are rated to a minimum of 1500 Vrms, due to the difference in ground potential between two buildings.
    – user71659
    Commented Jul 17 at 6:47

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